Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied as the fuel to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. As such these MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
Traditionally, the electrically conductive plates sandwiching the MEAs contain a reactant flow field for distributing the fuel cell's gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective anode and cathode (referred collectively herein as active area). These reactant flow fields conventionally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels.
The requirements for a well-performing flow field may be characterized into local requirements and global requirements. A local requirement generally applies to every point on the active area and a global requirement applies to the entire flow field design. To satisfy the local requirements of a well-performing flow field, the flow field should (1) deliver gas and humidification, (2) remove exhaust gases and (3) remove liquid water. To satisfy the global requirements of a well-performing flow field, the flow field should (4) satisfy local requirements at all points on the active area, (5) satisfy local requirements with a reasonably low overall pressure drop, (6) satisfy local requirements consistently over time thus producing stable flow, and (7) satisfy local requirements at all required flow and load conditions.
The requirement for stable flow (6), is a difficult requirement to meet. Two reasons may be cited for this difficulty. First, it is difficult to determine exactly when stable flow has been achieved because there is more than one condition under which it can be successfully accomplished. Stable flow requires the consistent removal of liquid water. However, water can be removed in more than one way. For example, in some cases gas velocity may be sufficiently high such that collection of liquid water is not possible. In other cases, liquid water may collect and then a pressure build up may occur, causing the liquid water to move out. In some cases low gas velocity and an inability to build pressure cause unfavorable water removal conditions and an unstable gas flow.
The second reason stable flow is difficult to meet is that in order to satisfy it, other flow field requirements must be compromised. For example, the aspects of a flow field design that satisfy requirements (3) and (6) directly compete with design aspects that satisfy requirements (4) and (5).
The following three examples demonstrate the difficulty in designing a flow field that can satisfy all requirements concurrently, including establishing either one of the two possible stable flow conditions needed for consistent water removal. In a first example, it is possible to achieve stable flow by establishing a high gas velocity condition. A high gas velocity condition is established by designing a flow path having a high pressure gradient. However, for an averaged-sized active area, the consequence of such a design is a flow field having an unacceptably high overall pressure drop. In this way, requirements (3), (4), and (6) are met while (5) is not satisfied.
In a second example, in order to reduce the overall pressure drop, Example 1 may be modified to have more parallel flow paths that are shorter in length. However, in reaching an acceptably low pressure drop, gas velocities become reduced to a level where liquid water is allowed to build up. Then, with the establishment of many parallel flow paths, removal of liquid water by a pressure build-up is no longer possible because a pressure build-up cannot be raised. Accordingly, requirements (4) and (5) are met while (3) and (6) remains unsatisfied.
In a third example, in order to facilitate liquid removal by a pressure build-up, Example 2 may be modified by taking away some of the parallel flow paths. However, if the requirement of a low overall pressure drop is to be maintained, the length that can be added to each flow path to compensate for removing flow paths is limited. In this case, all the requirements are met except the one requiring the flow field to cover the entire active area. Specifically, requirements (3), (5) and (6) are met and (4) is not satisfied.